In order to control nuclear reactivity, “FNR-Na” reactors use a neutron absorbing material containing boron carbide of simplified formula B4C.
This material is generally in the form of sintered cylindrical pellets stacked in a cladding, in order to form an absorbing element such as an absorber pin.
Under the combined action of temperature and irradiation, the initially massive boron carbide pellets may be degraded until cracks appear in the pellets.
During operation of the “FNR-Na” reactor, the sodium in the primary circuit is in liquid form and contains at least one radioactive substance. It circulates in the space between the boron carbide pellets and the cladding. Following degradation of the pellets, the liquid sodium contaminated with the radioactive substance may then penetrate into the cracks in the boron carbide pellets, or even along the fractures of the boron carbide pellets when the cracks have led to fragmented pellets. In the present description, the fractures are classed as cracks.
After stopping the reactor, the absorber pins are extracted from the reactor and then put in storage before treatment. The absorber pin then comprises cracked boron carbide pellets, in which the cracks contain sodium which is in solid form and is contaminated with at least one radioactive substance.
The contaminated absorber pin constitutes nuclear waste that poses a dual risk in terms of safety and security:                a chemical risk due to the residual sodium, which must be kept under an inert gas (such as argon or nitrogen) so that there is no risk of chemical reaction, for example with water or with the oxygen of the air. Depending on the conditions of storage before treatment, a proportion of the sodium at the surface may nevertheless be transformed, for example to soda and to hydrogen on contact with water, and in an uncontrolled manner;        a radiological risk due to contamination of the sodium with the radioactive substance, namely the radioactive isotopes from the primary circuit of the reactor.        
In order to be able to treat such nuclear waste through the conventional channels for removal of contaminated waste, it is firstly necessary to eliminate the chemical risk, i.e. to transform chemically or extract the contaminated metallic sodium present in the absorber pin, in particular in the cracks in the boron carbide pellets.
A method for chemical transformation of sodium by direct reaction between water and sodium is difficult to implement: it requires bringing these two chemical species into contact, but also requires control of the reaction kinetics, disposal of the soda and hydrogen produced, as well as absence of accumulation of reagents.
Now, sintered boron carbide is a chemically stable material of low porosity, the porosity generally representing less than 1% of the volume of the material. The sodium is therefore very confined within this material. Consequently, numerous cutting operations on the cladding would be required for treating the sodium-boron carbide radioactive mixture. However, this operation is long and difficult, as the hardness of the boron carbide pellets is such that it would damage and contaminate the cutting tools.
The presence of a radioactive substance also means working in a confinement enclosure under inert gas, such as a glove box. Now, the cutting operations are also difficult there, because of the difficulties of manipulation intrinsic to this type of enclosure. Moreover, they would generate dispersion of radioactive substance in the enclosure, which must be limited as far as possible.
The poor accessibility of the contaminated sodium therefore complicates its treatment as waste considerably.